1. Field of the Invention
This invention relates to methods of manufacture of beta-alumina.
2. Prior Art
Beta-alumina and its various derivatives are polycrystalline oxide ceramic materials that can be used as solid electrolytes in energy conversion devices using a liquid alkali metal. Such liquid alkali metal energy conversion devices are well known. In brief, they comprise a first electrode chamber containing a liquid alkali metal, a solid alkali cationically conductive impervious polycrystalline solid electrolyte separator membrane forming at least in part, the bounding wall of the first electrode chamber, and a second electrode chamber containing an electrode structure from which alkali metal ions can exchange from the solid electrolyte, which also forms in part the bounding wall of the second electrode chamber. A sodium sulphur cell is a typical example of such an alkali metal energy conversion device.
The chemical composition of beta-alumina is approximately 90% of aluminium oxide Al.sub.2 O.sub.3, with approximately 10% of the oxide of an alkali metal, typically sodium oxide, whose ions are diffusible with respect to the beta-alumina polycrystalline lattice (all percentage compositions in this specification and claims are percentages by weight). The structure of beta-alumina crystals is characterised by particular X-ray diffraction patterns, and at least four different types known as .beta., .beta.", .beta.''' and .beta."" have been described in the technical literature. The .beta. and the .beta." type appear to be the most useful modifications for solid electrolyte applications. Often a useful electrolyte is formed of a mixture of the .beta. and .beta. types. Small additions of dopants such as magnesium oxide or lithium oxide have been used to increase the formation of the more conductive BP variant. Mixed additions of lithium oxide and magnesium oxide have also been used.
The production of a polycrystalline beta-alumina article from a compact of powdered materials using a heating cycle is well known. The article is usually heated to a temperature in excess of 1000.degree. C., at a constant rate of temperature rise, typically 100.degree. to 200.degree. C. per hour. When the sintering temperature has been reached it is maintained for a period in excess of ten minutes, but often for a period of several hours and then allowed to cool. Cooling may be at the natural rate or may be set to a controlled rate using special control equipment.
More recently, for the sintering of alphaalumina, modified time temperature profiles have been used. Rather than maintaining a constant rate of temperature increase until the sintering temperature is reached, followed by a fixed interval at the sintering temperature the heating rate is progressively reduced as the sintering temperature is approached. This method of heating reduces the rate of densification of the ceramic compact. It is possible to use a transducer in physical contact with a powder compact to measure the rate of shrinkage, and to use the transducer in combination with electronic controllers to control the power input to the furnace. Using this technique the sintering process may be controlled with respect to the rate of densification and the total amount of densification that takes place. These methods have been used in place of the more usual methods where the rate of temperature increase and the time for which the maximum sintering temperature is maintained are controlled.
By coincidence there are similarities between the time/temperature cycles of rate controlled sintering and constant power heating schedules that are used commercially. In both cases the rate of temperature rise declines as the maximum temperature is approached.
More recently a new method of sintering has been described. See for example U.S. Pat. Nos. 3,903,225, 3,950,463, 4,059,663 and 4,070,542. This is known as fast pass-through sintering or fast zone sintering. This is used where the sintering is scheduled to take place in a short time interval, typically in less than 10 minutes. Often such sintering is completed in a time interval of less than 2 minutes. When such short time intervals are used it is not practical to sinter large amounts of material in a single batch. Heat transfer problems lead to difficulties with the specification of a precise time temperature profile throughout the bulk of large furnaces for short firing cycles. The material is passed continuously through a previously heated furnace. The furnace has an appropriately specified temperature profile between the entrance and the exit so that the articles pass through a prescribed heating and cooling schedule. Often the articles that are to be sintered are bigger in at least one dimension than is the heated zone of the furnace. Thus, as the articles pass through the furnace they experience a large temperature difference between different parts. Fast pass-through furnaces are most convenient for the manufacture of thin-walled ceramic tubes. Conveniently the tubes are moved axially through a tubular furnace, which has a heated zone at its centre. Often the temperature versus length profile of such tube furnaces is parabolic at the centre of the hot zone. Thus an article is subjected to a parabolic temperature versus time cycle when it is passed through the furnace. The rate of heating decreases progressively as the maximum temperature is approached. Conveniently the furnace tube is rotated and this can be used to transmit rotation to the article that is being sintered as it passes through the furnace. Rotation means may also be provided by known methods at the entry and exit when the tubes that are being sintered are about the same length as the furnace tube.
The use of multiple heating cycles is known in the manufacture of oxide ceramics. Bisque firing prior to sintering is a known practice in the manufacture of certain hard paste procelain. This heating is carried out before the sintering heat cycle. It is used in part to eliminate volatiles. Re-heating subsequent to sintering is also known in the art of pottery manufacture for example, during glazing. In this case multiple heating may be used to obtain special effects.
Multiple heating cycles are also known during the production and subsequent use of polycrystalline oxide ceramic solid electrolytes in energy conversion applications. Prior to fast firing, ceramic tubes may be subjected to a bisque firing cycle up to about 1000.degree. C. to remove volatiles, and to improve the strength of the ceramic compact prior to sintering in a rapid pass-through furnace. Subsequent to sintering the solid electrolyte tube may be re-heated and subjected to an annealing schedule for a period of between 1 hour and 24 hours at a temperature at least 50.degree. C. below the normal sintering temperature.
However, in all known methods of manufacturing beta-alumina ceramic articles the densification of the powder compact to form the impermeable polycrystalline cationically conductive ceramic article takes place in the course of one heating and cooling cycle in which the article is monotonically heated to a maximum temperature, held at a maximum temperature and then cooled subsequently. All of the shrinkage that accompanies the densification takes place in a single cycle of heating followed by cooling.
Although good quality electrolytes can be produced by the known methods that have been described, there are problems relating to the maintenance of a uniform grain structure. Beta-alumina ceramic articles are particularly susceptible to the development of inferior, duplex, grain structures in which larger elongated grains are formed within the matrix of fine grains. By close control of the process variables the methods that have been described previously may be used, but it is difficult to obtain in practice the very fine control that is required. Even greater difficulties are encountered when the known methods are used to produce ceramic electrolyte with exceptionally low resistivity. When it is necessary to obtain a high power density in a solid electrolyte energy conversion device, it is helpful to minimise the electrolyte resistivity. It then becomes necessary to maximise the formation of the .beta." variant of beta-alumina during sintering. It is found that known methods are particularly difficult for the production of high conductivity materials, that have a fine uniform microstructure and a high durability in a cell.